The present invention relates to a method for reducing the effects of multipath propagation in a receiver.
The GPS system (Global Positioning System) is a positioning system that comprises more than 30 satellites, of which a maximum of 12 are simultaneously visible to a receiver Among other things, the satellites transmit information about the satellite""s orbit (Ephemeris data), as well as information about the time according to the satellite""s clock. A positioning receiver normally determines its position in by calculating the time of propagation of signals transmitted simultaneously from several satellites in the positioning system to the receiver. To determine its position, the receiver must typically receive the signal of at least four satellites within its sight.
Each satellite of the GPS system transmits a so-called L1 signal at a carrier frequency of 1575.42 MHz. This frequency is also denoted as 154f0, where f0=10.23 MHz. Additionally, the satellites transmit an L2 signal at a carrier frequency of 1227.6 MHz, i.e. 120f0. In the satellite, these signals are modulated with at least one pseudo-random sequence. The pseudo-random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. This modulation technique makes it possible to discriminate the signals transmitted from different satellites at the receiver, even though the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, the pseudo-random sequence used to modulate the L1 signal is e.g. a so-called C/A code (Coarse/Acquisition code), which is a Gold code. Each GPS satellite transmits a signal using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed using the polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible for different C/A codes to be produced with an identical code generator. The C/A codes are thus binary codes, whose chipping rate is 1.023 MHz in the GPS system. The C/A code comprises 1023 chips, wherein the duration of one repetition of the code is 1 ms. The carrier of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the xe2x80x9chealthxe2x80x9d of the satellite, its orbit, time data, etc.
During their operation, the satellites monitor the condition of their equipment. For example, the satellites may use so-called watch-dog operations to detect and report possible faults in the equipment. The errors and malfunctions may be transitory or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, different satellites can be weighted differently on the basis of the health data. Thus, it is possible to minimize the effect of errors on the measurements possibly caused by satellites which seem unreliable.
To detect the satellite signals and to identify the satellites, the receiver must perform synchronization, in which the receiver searches in turn for the signal of each satellite and attempts to synchronize and lock to the signal so that the data transmitted with the signal can be received and demodulated.
The positioning receiver must perform synchronization e.g. when the receiver is switched on and also in a situation where the receiver has not been able to receive the signal of any satellite for a long time. Such a situation can easily arise e.g. in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which reduces the strength of the signal arriving at the receiver. Moreover, in urban areas, buildings affect the signal to be received, and furthermore, so-called multipath propagation can occur, wherein the transmitted signal arrives at the receiver via different paths, e.g. directly from the satellite (line-of-sight) and also reflected from buildings. Multipath propagation causes the same signal to be received as several signals with different phases.
Multipath propagation may cause inaccuracies in the positioning e.g. because the distance travelled by a multipath propagated signal is not equal to the straight-line distance between the transmitter and the receiver. Thus, if the receiver cannot distinguish between the direct signal and multipath-propagated components, positioning accuracy will be reduced.
The amplitude of a multipath-propagated signal is influenced by several factors, such as the reflective properties of the surfaces causing reflection of the radio signals, the distance travelled by the signal, and the number of reflections. Multipath propagation changes continuously with time. In general, both movement of the receiver and movement of the satellites causes the strength, phase, number, etc. of the multipath-propagated signals arriving at the receiver to change continuously. Thus, changes are also caused in the signal received by the receiver. For this reason, the receiver must try, after synchronization, to remain continuously locked to the signal of each satellite from which information is received e.g. for positioning. In the receiver, the code phase is calculated very frequently, and if necessary the oscillator is adjusted in such a way that the receiver remains synchronized.
The positioning arrangement has two primary functions:
1. to calculate the pseudo-range between the receiver and the different GPS satellites, and
2. to determine the position of the receiver using the calculated pseudo-ranges and information about the position of the satellites. Information about the position each satellite can be calculated on the basis of the Ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudo-ranges, because the time is not accurately known at the receiver. Thus, determinations of position and time are repeated, until a sufficient accuracy has been achieved with respect to time and position. Because time is not known with absolute precision, the position and time must be determined by linearizing a set of equations for each new iteration. The pseudo-range can be calculated by measuring the mutual, apparent propagation delays of the different satellite signals.
Almost all known GPS receivers use correlation methods to calculate distances. In a positioning receiver, the pseudo-random sequences of different satellites are stored or generated locally. A received signal is down-converted to an intermediate frequency, after which the receiver multiplies the received signal with a stored pseudo-random sequence. The signal obtained as a result of the multiplication is integrated or low-pass filtered, wherein the result provides information about whether the received signal contained a signal transmitted by a satellite. The multiplication is repeated in the receiver so that each time, the phase of the pseudo-random sequence stored in the receiver is shifted. In other words, a cross-correlation is performed between the received signal and the pseudo-random sequence generated/stored in the receiver. In a situation where there is no multipath propagation, the correct code phase is deduced from the correlation result preferably in such a way that when the correlation result is the greatest, the correct code phase has been found. The receiver is then correctly synchronized with the received signal. On the other hand, in a situation where multipath propagation is present, it is more difficult to determine the correct code phase.
After synchronization with the code, fine tuning of the frequency and phase-locking are performed. The correlation result also reveals the information transmitted in the GPS signal; that is, the signal in question is a demodulated signal.
The above-mentioned synchronization and frequency tuning process must be repeated for each satellite signal received at the receiver. Consequently, this process consumes a considerable amount of time, particularly in a situation where the signals to be received are weak. To accelerate this process, some prior art receivers use several correlators, making it possible to search for several correlation peaks at the same time.
In prior art receivers, attempts have been also made to reduce the influence of multipath propagation, primarily on the basis of three different principles. The first principle is based on filtering using filters having a long time constant. This is based on the assumption that the expected value of the estimation error caused by multipath propagation is zero. However, this is not completely correct, and therefore, even an estimate filtered for a long time remains inaccurate. According to the second principle, additional correlators are used to determine the cross-correlation function of the received signal and the locally generated code, wherein it is possible to evaluate the influence of multipath propagation on the received signal on the basis of features of the correlation function. In the third principle, three correlators located within a narrow code phase range are used, placed at intervals of e.g. 0.1 chips. On the basis of the information formed by these three correlators, an attempt is made to determine the code phase of the signal. United States patent U.S. Pat. No. 5,615,232 discloses a solution that applies said second principle.
It is an aim of the present invention to provide a receiver in which the effect of multipath propagation is reduced enabling a directly received signal to be identified from multipath propagated signals. The invention is particularly suitable for use in positioning receivers, but is also applicable in other receivers, preferably CDMA receivers, where the receiver must be synchronized and locked to a spread spectrum signal. The invention is based on performing a frequency domain deconvolution on the signals formed by several correlators located within a narrow code phase range which represent a sampled cross-correlation function. The deconvolution is performed using a cross-correlation function model free of multipath propagation, and after this the code phase resolution is improved by interpolation. Deconvolution is performed using the following approach: samples of the cross-correlation function are transformed into the frequency domain and are divided by a frequency domain model of the cross-correlation function, in such a way that division by zero is prevented by replacing values having an absolute value less than a certain threshold value with said threshold value. Interpolation is performed by adding filler values, preferably zeros, to the frequency domain representation of the sample vector before transformation back to the time domain. In a method according to a preferred embodiment of the invention, a frequency analysis step is also performed, in which an analysis matrix formed by the time-to-frequency transform is analysed. This makes it possible to detect both the code phase and the frequency deviation of a multipath-propagated signal.
The present invention provides considerable advantages compared with prior art methods and receivers. Using the method according to the invention, the detection of multipath-propagated signals is improved, particularly in situations where the multipath-propagated signal is delayed only slightly compared with the arrival of a directly propagated signal at the receiver, that is, if the phase difference between the directly propagated signal and the multipath-propagated signal is small. Using the method according to the invention, more reliable synchronization with the desired signal can be maintained, particularly in positioning receivers, wherein more accurate positioning is achieved. Furthermore, the receiver can be implemented with a smaller number of correlators than prior art receivers, while still achieving equally good resolution. A receiver according to the invention can be implemented with a relatively small number of components and the total energy consumption can be kept at a reasonable level compared with a prior art receiver capable of achieving the same resolution, wherein the invention is particularly well suited for use in portable devices. Thus, the positioning receiver can also be implemented in connection with a wireless communication device,